1. Field of the Invention
The present invention relates to a printer using a print head and a paper feed controller for controlling paper feed means for feeding a printing medium such as printing paper.
2. Description of the Related Art
In recent years, a great advance has been made in terms of image quality of an ink-jet printer. However, there is a continuing need for a further improvement in resolution which is one of factors of image quality. In a serial-type ink-jet printer, a carriage including a print head is scanned across a printing medium (hereinafter, also referred to as printing paper or simply as paper) while emitting ink with well controlled timing thereby forming an image. Each time the carriage is scanned across the paper, the paper is fed a predetermined distance.
The resolution of a printed image depends on carriage scanning resolution (resolution in the main scanning direction) and paper feed resolution (resolution in the sub scanning direction).
In order to achieve low operating noise and high accuracy in ink emission timing, the scanning of the carriage is generally performed by means of feedback control using a DC motor and an encoder. When low cost is more important than high performance, the scanning of the carriage is performed by means of feedfoward control using a pulse motor. In the feedback control, the ink emission timing is accurately controlled on the basis of encoder pulses. On the other hand, in the feedfoward control, on the assumption that the carriage moves at a constant velocity, the ink emission timing is determined on the basis of an applied pulse signal or a clock pulse signal.
The feeding of paper is also performed by means of feedback control using a DC motor and an encoder to achieve low operation noise and a high feeding speed.
In the feeding of paper, it is required to stop the paper when printing is performed. In the conventional technique, to precisely feed paper by a distance in small and precise units (with high resolution) to a next stop position, the resolution of the encoder is increased or the nozzle pitch of the print head is reduced. When neither the increasing of the resolution of the encoder nor the reducing of the nozzle pitch is possible, the stop position is determined such that the distance to the next stop position becomes equal to a common multiple of the minimum paper feeding unit and the nozzle pitch of the print head.
Regarding the increase in resolution of the encoder, a high-resolution encoder system used in industrial applications is expensive and thus unsuitable for use in general applications that need low-cost printers. Thus, in general, an encoder sensor with a resolution of 150 to 360 Lpi is used, and the ratio of the diameter of an encoder wheel to the diameter or a paper feed roller is set to be large enough to achieve high resolution.
It is also known to improve the accuracy of the stop position such that when the position reaches a point a predetermined distance before a next stop position, a calculation is performed to determine the time at which to turn off the electric power supplied to a motor. When the calculated time has elapsed, the electric power to the motor is turned off.
However, to improve the paper feed resolution of the conventional printer, many problems must be solved. Regarding the reduction in the nozzle pitch of the print head, the reduction is limited by limitations on the head design and production techniques and the upper limit of cost, and thus the improvement in resolution is not easy (in particular, the improvement needs a very large increase in cost).
In the technique of determining the stop position such that the distance to the next stop position becomes equal to a common multiple of the minimum paper feeding unit and the nozzle pitch of the print head, the paper feeding distance does not have a simple value (deviated from 2 raised to nth power) and thus a complicated calculation is needed in image processing. Furthermore, all nozzles are not equally used, but particular nozzles are used more frequently than the other nozzles. This can cause a reduction in performance of the print head. A further problem of this technique is that a restriction is imposed on the design of the diameter of the paper feed roller and the diameter of the encoder wheel.
To improve the accuracy of the stop position, it is needed to acquire a large amount of information very shortly before the paper feed roller is stopped. If the stop position is controlled such that it is located at a position exactly corresponding to a slit signal (a rising or falling edge of phase A or B), high accuracy in the stop position can be achieved. However, in this technique, the resolution is limited to 2 raised to nth power such as 1200 dpi, 2400 dpi, 4800 dpi and so on.
On the other hand, to increase the amount of information obtained very shortly before the stop position, it is needed to increase the size of the encoder wheel connected to the paper feed roller, which results in an increase in the total size of the printer. Besides, if the amount of information is increased, it becomes necessary to process the large amount of information, which can cause an increase in processing time and a reduction in throughput.
If the reduction in the total size or the increase in operating speed is given higher priority in the design of the printer, the amount of information becomes smaller (for example, by a factor of ½, due to the limitation of the resolution to 2 raised to nth power) than can be obtained when the amount of information is given higher priority in the design. This can make it impossible to obtain information necessary in a low-velocity region very shortly before the paper feed roller is stopped. The lack of a sufficient amount of information makes it impossible to precisely determine the velocity immediately before the stop position and thus the stop position accuracy becomes low.
For example, when the print head has a nozzle pitch of 1200 dpi, resolution of 1200 dpi can be achieved by controlling the paper feed roller based on a single edge/single phase control scheme, resolution of 2400 dpi can be achieved using a two edge/single phase control scheme, and resolution of 4800 dpi can be achieved using a two edge/two phase control scheme, and thus all resolutions of 1200 dpi, 2400 dpi, and 4800 dpi can be achieved in the paper feed direction.
However, information can be acquired only at intervals of 4800 dpi in a very short period immediately before paper is stopped. Because the paper feeding velocity is very low in the very short period immediately before the paper is stopped, information can be obtained a very small number of times. For example, when the servo control period of the feedback control is 1 ms, information is acquired only once when a servo interrupt occurs, even in the two edge/two phase control scheme. In such a situation, the accuracy of the calculated velocity becomes low. Thus, controlling of the feeding of paper in the very low velocity range is difficult. This makes it difficult to precisely control the stop position.
If the paper feed roller can be controlled using the single edge/single phase control scheme with a resolution of 2400 dpi, a resolution of 4800 dpi can be realized using the two edge/single phase control scheme. However, although high accuracy twice that obtained in the previous example is achieved, the diameter of the encoder wheel becomes twice as large as that in the previous example, and thus it becomes difficult to achieve a small total size. Besides, the encoder wheel cannot be rotated at a high speed unless the sensor has a correspondingly high response speed.
It is known to increase the stop position accuracy by performing a stop operation such that when a predetermined distance before a target stop position is reached, a calculation is performed as to the time when to perform the stop operation, and electric power to a motor is turned off when the calculated time has elapsed. However, in this technique, the maximum stop position error is not guaranteed, and thus this technique cannot be used when high stop position accuracy is required and the maximum stop position error must be guaranteed.
In general, because the velocity in the low velocity range is calculated based on a small number of encoder pulses, it is difficult to achieve high accuracy in the calculated velocity. Besides, the velocity can fluctuate due to a disturbance such as mechanical friction, a back tension of paper, a fluctuation of a driving force transmission load, or cogging of a motor.
In practice, as described above, in the above-described technique of turning off the electric power to the motor at the time calculated based on the velocity calculated at the position the predetermined distance before the target stop position, it is difficult to precisely calculate the timing of turning off the electric power to the motor. Besides, a varying deviation of the stop position from the target stop position occurs after the electric power to the motor is turned off, owing to a disturbance before the paper feed roller stops, such as the back tension of paper, the fluctuation of the driving force transmission load, or cogging of the motor.